Corrosion (B2-Durability) covers considerations for continued performance of roof and wall cladding over the building lifecycle.
Key topics include:
Source: New Zealand Building Code, Clause B2 Durability
The objective of this provision is to ensure that a building will continue to satisfy the other objectives of this code throughout its life.
Building materials, components and construction methods shall be sufficiently durable to ensure that the building, without reconstruction or major renovation, satisfies the other functional requirements of the NZBC throughout the life of the building
Building elements must, with only normal maintenance, continue to satisfy the performance requirements of the NZBC for the lesser of the specified intended life of the building, if stated, or:
- The life of the building, being not less than 50 years, if:
- those building elements (including floors, walls, and fixings) provide structural stability to the building; or
- those building elements are difficult to access or replace; or
- failure of those building elements to comply with the building code would go undetected during both normal use and maintenance of the building.
- 15 years if:
- those building elements (including the building envelope, exposed plumbing in the subfloor space, and in-built chimneys and flues) are moderately difficult to access or replace; or
- failure of those building elements to comply with the building code would go undetected during normal use of the building, but would be easily detected during normal maintenance.
- 5 years if:
- the building elements (including services, linings, renewable protective coatings, and fixtures) are easy to access and replace; and
- failure of those building elements to comply with the building code would be easily detected during normal use of the building.
Individual building elements which are components of a building system and are difficult to access or replace must either:
- all have the same durability; or
- be installed in a manner that permits the replacement of building elements of lesser durability without removing building elements that have greater durability and are not specifically designed for removal and replacement.
NZBC Clause B2, Durability, requires fifteen years to perforation for claddings easily accessed for replacement. Fifteen years is also required for internal gutters and downpipes, and five years for external gutters.
NZBC B2 requires 50 years’ durability for flashings that require the removal of cladding above to be replaced, while table 20 of NZBC E2/AS1 only requires 15 years’ durability for such flashings. The COP recommends the higher figure as good trade practice and in many cases, lower life-cycle costing.
Generally, higher durability than the minimum requirements can be achieved by using materials and methods outlined in this COP, with no maintenance of coatings other than washing areas which are not naturally washed by rain. Elements more difficult to replace, or to access for maintenance, should be constructed of more durable material.
Normal Maintenance means work recognised as being necessary to achieve the expected durability of a given building element.
According to B2/AS1, normal maintenance may include:
Although roof or wall cladding can be easily accessed and therefore easily replaced, the same cannot be assumed for flashings. Flashings might be embedded in plaster or behind other building elements, making them hard to replace without removing cladding or other building features such as windows.
Cladding material may be described as hidden, sheltered or exposed. Some flashings may have sections falling into all these categories, in which case the worst case (sheltered) should prevail in material selection.
Good design, correct selection of materials, and good installation and maintenance practices are required to achieve optimum product lifespan.
Corrosion is the process by which something erodes because of a chemical reaction.
Metal corrosion is a reaction of metal with its environment that causes measurable alteration and is part of metal's inherent tendency to revert to an original, more stable form. The red rusting of iron and steel is a visible example of corrosion and other examples include the weathering of copper and the oxidation of aluminium and zinc.
Corrosion can only happen in the presence of an electrolyte, e.g., water. The occurrence of salt (or other contaminants) in the water increases the conductivity of the electrolyte and therefore greatly increases the reaction rate.
Salt contamination will also affect the time of wetness. On a clean surface, water vapour will condense at 100% relative humidity, on a salt-covered surface, a wet film can be formed at a relative humidity level of 75% and more.
Corrosion can also be the result of direct contact with another metal or substance, or the result of run-off from incompatible surfaces or fall-out of corrosive particles. Time of wetness, presence or lack of oxygen, and atmospheric contaminants greatly affect the rate of corrosion.
Differences in electrical potential on the surface of corroding metal create microscopic cells comprising cathodes and anodes. In the case of iron, the positively charged electrons in the anode react with the negatively charged hydroxyl ions in the electrolyte to form iron oxide on the anode. Similar reactions occur with other metals. Polarisation changes on the surface cause anodic areas to become cathodic and vice versa, so that over time the rate of corrosion is relatively uniform over the surface.
The build-up of debris on a cladding surface will promote corrosion. The salts in the debris react with the cladding each time they are wetted, and the deposits themselves impede surface-drying, increasing the time of wetness.
To understand metal performance in any specific environment, the unique properties of each metal should be considered in conjunction with other metals it is used with.
All metals react differently to the atmosphere and to any contaminants that come into contact with their surface by rain, wind or condensation. The indicator of acidity or alkalinity is pH, measured from 0 – 14; acidity/alkalinity is an important corrosion factor. pH 7 is neutral, below 7 is more acidic, above 7 is more alkaline.
As can be seen from the graph above, zinc performs better in alkaline environments, and aluminium performs better in acidic environments.
Aluminium-zinc coatings should be avoided in buildings such as closed animal shelters or fertiliser storage sheds, where alkalinity may be high.
The zinc and aluminium families of metallic coatings protect the steel base from corrosion in two different ways:
Zinc is more electrically active than steel. By coating steel with zinc, or a zinc-rich product, the zinc becomes the anode for the steel. The steel then becomes the cathode and does not react with the electrolyte. The process is known as cathodic protection.
This protective effect occurs even when there is a small area of steel exposed directly to the electrolyte, such as a cut sheet edge, drill hole or scratch.
While the zinc reacts in preference to the steel, it does so at a slower rate. In normal environmental conditions, the zinc-oxide layer that initially forms on the surface of the zinc combines with carbon dioxide in the atmosphere to form zinc carbonate. That creates a sealed layer with excellent adhesion, and as zinc carbonate has very low solubility, reaction with the electrolyte slows even more.
Barrier protection works primarily by providing a physical barrier between the atmosphere and the steel substrate.
The surface of aluminium-dominant coatings is initially very active, but it quickly forms an inert aluminium-oxide film when exposed to normal atmospheric conditions. Aluminium dominant coatings on steel mainly provide barrier protection as the aluminium, having formed an oxide surface, ceases to offer substantial sacrificial protection.
The exposed edges of barrier protected cladding should not be in contact with corrosive surfaces. See 4.9.4 Compatibility Table
The durability performance of metal roof and wall cladding depends on the macro- and microclimates, airborne contaminants, and the material itself.
The macroclimate is the general environmental category where the building is situated.
Designers should be aware of macro- and microclimates and the degree of contamination. They should design their building and select materials considering a combination of all these factors.
The major contributor to metal corrosion in New Zealand is sea spray. Sea spray contains a mixture of salts consisting of 2.5 to 4% sodium chloride and small quantities of magnesium, calcium and potassium chloride. These salts make water far more electrically conductive.
Sea spray, evaporation, and infrequent rain increase salt concentrations on exterior surfaces, particularly when it accumulates in unwashed areas.
The distance airborne salt is carried inland varies significantly with local wind patterns. Salt deposits have been measured as far inland as Lake Taupo in the North Island. Geographic or man-made obstructions, such as trees or buildings, slow air velocity and allow the air to discharge some of its salt burden, which can make the environment less aggressive. Conversely, where there are few impediments to the free flow of air, severe marine influence can extend well inland.
In high humidity levels, or when wetted by condensation, marine salts absorb water and form a chloride solution. Therefore, the effect of salt spray is greatest in unwashed areas, where salts can accumulate over time.
Where the ends of roof cladding are exposed to contaminants such as sea salt or industrial pollutants, it is good practice to provide an over flashing which discharges into the gutter or spouting. (See 8.5.5.4A Gutter-Eaves Flashing.)
The over flashing should extend 50 mm into the gutter, and the underlay finishes on the down-slope of the flashing. If there is no over-flashing to the gutter, the underlay should be extended into the gutter by a minimum of 20 mm.
In some cases, the over flashing becomes a sacrificial flashing which can extend the life of the cladding. In such circumstances, the COP recommends making the flashing from aluminium.
Suppliers of pre-painted metal offer alternative products for different environments, using different metallic coatings, paint systems, paint thickness and metals. The designer or the roof cladding contractor should carefully assess and evaluate these options to comply with the NZBC.
The boundaries of different corrosion zones are difficult to define because many factors determine the corrosivity of a particular location. The designer should choose the appropriate materials for the location, which meet the minimum durability requirements of the NZBC and satisfy customer expectations.
Wind is responsible for the salinity present in marine atmospheres. The wind picks up particles of salt from breaking waves and can carry them inland. The quantity of salt aerosol entrained by the wind is affected by many factors, such as wind strength, wave height, the width of the generation zone, and the contours of the seabed and coastline. These factors along with the persistence of the wind from a given quarter determine the corrosivity of a shoreline.
While salt deposits are measurably present in inland areas such as Taupo, the main effect of marine atmospheres reaches just a few hundred metres from the shore. Particles of salt in the air deposit on adjacent surfaces through gravity and contact; the rate at which deposits settle is affected by the roughness of the ground that the salt-laden air passes over. Obstacles such as trees slow the wind down, increasing the rate of gravitational deposit, and bringing the salt aerosol in more contact with surfaces on which they can deposit.
On the other hand, open flat land and natural “wind tunnels” can allow quite high concentrations of salt to travel several hundred metres inland.
A site’s location, relative to the sea or marine inlets, is a common method used to assess the corrosivity of a location. The distance from salt water for a given Zone varies with the location, depending on the prevailing winds and roughness of water in those areas, as well as the evenness of the terrain it passes over.
Where environmental Zones overlap, a site-specific evaluation may help define the category into which it best fits. Visual evidence of corrosion on adjacent metal surfaces may be present, ground roughness can be assessed, industrial influences can be evaluated and data about the persistence of onshore winds can be obtained from NIWA.
More local factors that affect the corrosivity of a specific location include:
There are many ways of more accurately determining the actual corrosivity of a given location. The most commonly accepted method as outlined in ISO 9223 is measuring first-year corrosion rate of different metals: mild steel (MS), zinc, aluminium and copper. The COP uses the first-year corrosion rate of mild steel as the most relevant and reliable indicator of a location’s corrosivity.
The names given by different Standards for given corrosion zones vary. The Corrosion Zones in the Code of Practice are similar to those published in NZS 3604:2011 except that:
| NZS3604 | Code of Practice | Description | MS Corrosion Rate (g/m²) |
|---|---|---|---|
| B | A (Mild) | Far inland, with no industrial pollution or thermal activity, or dry internal. This condition is not commonly found externally in New Zealand. | 1 – 10 |
| B (Moderate Inland) | Most dry rural areas in New Zealand, 50 km from the coast, are in this category. It can extend closer to the coastline of sheltered water in low rainfall areas. | 10 – 80 | |
| C | C (Moderate Marine) | This category covers area of low marine influence. It can extend from 50 km inland to within 1 – 1.5 km of west coast beaches, or be in the immediate vicinity of calm estuaries. | 80 – 200 |
| D | D (Severe Marine) | In this category, marine influences are frequently apparent. Its proximity to the coast is determined by the roughness of the water, prevailing winds, ground roughness and sheltering. | 200 – 400 |
| E (Very Severe Marine) | In this category, the structure is normally exposed and marine influences are almost constantly apparent. | 400 – 650 | |
| F (Extreme Marine) | This category is rare in a building site. It would be an exposed location very close to breaking surf. | 650 – 1 000 |
The following tables provide guidance for specific regions and environments. See 4.6.1A Corrosion Zone Categorisation and First year Mild Steel Corrosion Rate (g/m²) for descriptions of specific categories.
Note: this is the minimal requirement to achieve compliance with NZBC Clause B2-Durabilty. Meeting the minimum requirements of NZBC clause B2 Durability does not necessarily represent optimal product selection. In a transition zone, it may be more cost effective over the life cycle of the building, and for meeting customer expectations, to choose a more durable option.
| Marine Zone | Exposed Fastener Class (minimum) | Acceptable Materials | |
|---|---|---|---|
| ***As defined by AS/NZS 2728. | |||
| B: Moderate Inland | C4 |
| |
| C: Moderate Marine | C4 |
| |
| D: Severe Marine | C4 |
| |
| E: Very Severe Marine | C5 |
| |
| F: Extreme Marine | C6 |
| |
| Marine Zone | Exposed Fastener Class (minimum) | Acceptable Materials | |
|---|---|---|---|
| *The practicality of carrying out regular maintenance, and difficulty of replacement, should also be considered when considering wall cladding material options. | |||
| ***As defined by AS/NZS 2728. | |||
| B: Moderate Inland | C4 |
| |
| C: Moderate Marine | C4 |
| |
| D: Severe Marine | C4 |
| |
| E: Very Severe Marine | C5 |
| |
| F: Extreme Marine | C6 |
| |
| Marine Zone | Acceptable Materials | |
|---|---|---|
| **Stainless steel must not be in wet contact with metallic coated steel, plain or painted. | ||
| ***As defined by AS/NZS 2728. | ||
| B: Moderate Inland |
| |
| C: Moderate Marine |
| |
| D: Severe Marine |
| |
| E: Very Severe Marine |
| |
| F: Extreme Marine |
| |
In areas where humidity or local conditions create an increased likelihood of corrosion, special consideration should be given to the specification and use of metal roof and wall cladding and accessories.
The West Coast is characterised by high rainfall and a very severe coastal environment between the sea and the Southern Alps; many households have coal burning fires that produce sulphur dioxide that is detrimental to metals.The combination of these factors means that either a shorter performance life should be accepted or the use of more durable metals and coatings considered.
The North Island’s south coast not only has strong onshore prevailing winds, but strong current flows increase wave action and the amount of salt-laden air, creating a particularly harsh marine environment.
Buildings within 50 m of a geothermal fumarole are considered to have a geothermal microclimate, which causes increased corrosion due to higher humidity levels combined with hydrogen sulphide.
Highly active geothermal areas, such as much of Rotorua, are considered geothermal, even in the absence of a local fumarole.
Corrosive internal environments with high humidity, causing condensation, and pollutants generated within the building can also affect neighbouring buildings. These include:
Sulphur dioxide develops when burning fossil fuels. After oxidation and reaction with water it forms sulphuric acid (H2SO4) that can contribute considerably to the atmospheric corrosion of zinc and steel.
Burning resinous woods, CCA-treated timber, low-grade coal or oils with a high sulphur content can increase the fall-out deposit or condensate from flue gas.
Exhaust fans can cause similar problems when corrosive gases are not filtered at their source.
New Zealand has a harsh level of ultraviolet light. Paint formulations that are successful overseas have sometimes been found lacking colour fastness in the Australasian environment. Proprietary pre-painted cladding products should give durability protection and keep its appearance for at least 15 years, and at the end of that period still have the anti-corrosive primers intact and present a good substrate for over-painting.
The performance of all paint coatings, however, can be affected by avoidable outside influences.
Colour match paint is sold in pre-colour matching accessories, such as soft edging or brackets, to match the pre-painted cladding. It should only be pre-applied before installation and should not be used to repair minor scratches and blemishes. These paints may closely match the pre-painted surface initially, but being an air drying acrylic, it is likely to weather differently to the pre-applied coating and cause unsightly blotches.
Sun screen lotions containing semi conducting metal oxides, such as titanium dioxide and zinc oxide, will cause discoloration of painted surfaces over time. There is no cure for such damage so contact of such chemicals with pre-painted cladding must be avoided.
Most Graffiti removal processes involve the use of strong solvents, abrasion, or extreme temperatures. It is generally difficult to remove graffiti without compromising an existing organic finish; the normal remedy is to overpaint as soon as possible. On a weathered pre-painted finish this can generally be achieved by washing and rinsing the surface to remove dirt and other contaminants, then applying two coats of acrylic paint.
It should be noted that such overpaint cannot over time, be expected to match the appearance of existing pre-applied finishes.
Warm temperatures, dust, and rainfall can create an environment for lichens to flourish. Over time the root structures of lichen may infiltrate a painted surface and cause permanent damage.
Lichen growths retain moisture and, therefore, increase the time of wetness. They are fed by nutrients in the atmosphere and tend to occur more commonly in moist and unpolluted environments. Where lichen growth is present, it should be removed. See 16 Maintenance .
Physical removal is difficult to achieve completely, and recolonisation is usually rapid. Chemical treatment is recommended. New Zealand Steel publishes a formula for batching a 2% sodium Hypochlorite solution to be used for this.
Materials comprising the building envelope should not be considered in isolation, as their performance can be affected by contact with or run-off from other materials.
This reaction is caused by either their relative places in the electro-chemical series or by the mineral composition of their surface moisture.
A component which may appear suitable may prove unsatisfactory in service because it is incompatible with another material or substance in contact with it.
This incompatibility can occur when the metals are in electrolytic contact or when water from one metallic surface discharges onto another. When a noble metal dissolves in water and flows over a less noble one, the more noble metal deposits on the less noble metal and create corrosion conditions.
| Magnesium | Active (Anode) |
| Zinc | |
| Galvanised Steel | |
| Aluminium | |
| Mild Steel | |
| Cast Iron | |
| Lead | |
| Brass | |
| Copper | |
| Bronze | |
| Monel | |
| Nickel (passive) | |
| Stainless Steel 304 (passive) | |
| Stainless Steel 316 (passive) | |
| Silver | |
| Titanium | |
| Gold | |
| Graphite | |
| Platinum | Noble (Cathode) |
| The similarity of metals is indicated by their relative position in the galvanic series. The more dissimilar the metals, the greater the corrosion potential in a galvanic circuit | |
Generally, run-off from metals higher on the table to those lower will not cause corrosion, but run-off in the other direction may do so.
Metals such as aluminium, stainless steel, and Zincalume® form an inert surface that does not produce soluble salts and run-off from them will not result in dissimilar metal corrosion. However, because these surfaces are inert, potential for run-off to create inert catchment corrosion on unpainted zinc or galvanised steel must be considered.
Where the use of dissimilar metals is unavoidable, a non-absorbent inert material can be used as an electrolytic separator. Long-term corrosion resistance depends on the separation remaining effective.
Examples of separation materials are inert plastic tapes, polythene or silicone sealant, and in the case of fasteners an EPDM sealing washer.
Where gutters and spouting are made from materials incompatible with roof the cladding, there can be contamination from immersion of the sheet ends if the gutter is poorly drained. Special provisions, such as ensuring there is a 10 mm drainage gap between spouting and fascia should be made to avoid immersion of coated steel roofing into copper gutters. Discharging the gutter into a rain head with leaf deflector can also help. Low front spouting can be considered, but that creates aesthetic issues and may contribute to early corrosion in marine areas. No part of copper gutters should be in contact with coated steel roofing or flashings.
The electrochemical tables or galvanic series scales, often quoted in technical literature as a measure of corrosion, show the electro-potential between pure metals, not between their oxides, carbonates or chlorides.
Although theoretically correct, these tables can give a misleading indication of the performance of different materials in contact.
This series only applies to pure metal. Under certain conditions, some metals react with the environment or chemicals to form a passive surface, which renders them less active, so that any ranking can be misleading. "Passivity" becomes an important phenomenon in controlling corrosion rates.
However, the Electrochemical series table is still a useful indicator of electrode potential. The further apart two metals are in the electrochemical series, the greater the potential difference between them.
Metals termed anodic, active, negative, or less noble corrode in preference to metals that are deemed more cathodic, noble, positive or less active. The less noble metal becomes the anode and is subject to corrosion. The greater the potential difference, the more corrosion there will be of the less noble metal; i.e., on the anode.
The difference in nobility is why zinc can protect a steel substrate.
Different electrolytes can lead to different rankings, and metal alloys may display more than one potential than that which applies to their "active" state.
The exposed surface ratio of anode and cathode determines the rate of dissimilar metal corrosion. For instance, if a fastener which has a small surface compared to the cladding becomes the anode, its current density will be high, and the fastener will corrode quickly; e.g., an aluminium rivet in a copper sheet. When the opposite is the case, the effect is not so great.
The 4.9.2A Electrochemical table shows zinc is more active than steel. Contact between steel and zinc, in the presence of moisture, will cause the zinc to corrode or sacrifice itself, to protect the steel.
Timber is generally acidic, although some timbers—such as cedar—are more acidic than others. The interaction between preservative treated timber and metal depends on the moisture content of the timber, the time of wetness and the type of treatment. The corrosion rates of metal in contact with wet CCA-treated timber and with untreated radiata are similar.
Neither AZ coated steel nor aluminium should be used as a flashing embedded in concrete or masonry.
The compatibility table should be regarded as indicative only, due to the many permutations of the environment, the amount of moisture present and the relative size of the components.
The indicator for “use with caution in moderate environments” should be interpreted as a warning that it could be unsuitable when there is a risk of continued moisture or other contaminants.
This online tool interactively provides an interpretation of the information in the 4.9.4B Material Compatibility Table. Simply select the two materials in use to view compatibility.
The Code of Practice Online provides an interactive tool to interpretation of the information in the 4.9.4B Material Compatibility Table Table, by simply selecting the two materials in use to view compatibility. This tool is only available online at www.metalroofing.org.nz/cop/durability/compatibility#compatibility-table...
Material water flows from | Aluminium | prepainted aluminium | AZ Coated Steel | zinc | Galvanised steel | Painted Galvanised Steel | Pre-Painted Steel | Copper/brass | Stainless steel | Lead | Plastic/glass | concrete/plaster | Wet timber | Cedar | Butyl rubber | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Aluminium | Contact | Y | Y | Y | Y | Y | Y | Y | N | C | N | Y | N | N | N | C |
| Run onto | Y | Y | Y | N | N | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| prepainted aluminium | Contact | Y | Y | Y | Y | Y | Y | Y | N | N | N | Y | N | N | N | N |
| Run onto | Y | Y | Y | N | N | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| AZ Coated Steel | Contact | Y | Y | Y | Y | Y | Y | Y | N | N | N | Y | N | N | N | N |
| Run onto | Y | Y | Y | N | N | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| Zinc | Contact | Y | Y | Y | Y | Y | Y | Y | N | C | Y | Y | C | N | N | N |
| Run onto | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| Galvanised steel | Contact | Y | Y | Y | Y | Y | Y | Y | N | C | Y | Y | Y | N | N | N |
| Run onto | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| Painted Galvanised Steel | Contact | Y | Y | Y | Y | Y | Y | Y | N | N | N | Y | N | N | N | N |
| Run onto | Y | Y | Y | N | N | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| Pre-Painted Steel | Contact | Y | Y | Y | Y | Y | Y | Y | N | N | N | Y | N | N | N | N |
| Run onto | Y | Y | Y | N | N | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| Copper/brass* | Contact | N | N | N | N | N | N | N | Y | Y | C | Y | C | Y | Y | Y |
| Run onto | N | N | N | N | N | N | N | Y | Y | C | Y | Y | Y | Y | Y | |
| Stainless steel | Contact | C | N | N | C | C | N | N | Y | Y | Y | Y | Y | Y | Y | Y |
| Run onto | Y | Y | Y | N | N | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| Lead* | Contact | N | N | N | Y | Y | N | N | C | Y | Y | Y | Y | N | Y | Y |
| Run onto | C | N | N | Y | Y | N | N | Y | Y | Y | N | C | Y | Y | Y | |
| Plastic/glass | Contact | Y | Y | Y | C | C | Y | Y | Y | Y | Y | Y | C | Y | Y | Y |
| Run onto | Y | Y | Y | N | N | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| Concrete/plaster | Contact | N | N | N | C | C | N | N | C | Y | Y | Y | Y | Y | Y | Y |
| Run onto | N | C | N | Y | Y | N | N | Y | Y | Y | Y | Y | Y | Y | Y | |
| Wet timber | Contact | N | N | N | N | N | N | N | C | C | N | Y | Y | Y | Y | Y |
| Run onto | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| Steel* | Contact | C | N | C | N | N | C | C | N | N | C | Y | C | N | N | Y |
| Run onto | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| Cedar* | Contact | N | N | N | N | N | N | N | Y | Y | Y | Y | Y | Y | Y | Y |
| Run onto | Y | Y | C | C | C | Y | Y | Y | Y | Y | Y | Y | Y | Y | Y | |
| Butyl rubber | Contact | C | N | N | N | N | N | N | Y | Y | Y | Y | Y | Y | Y | Y |
| Run onto | C | C | N | N | N | C | C | Y | Y | Y | Y | Y | Y | Y | Y | |
| Y | Suitable |
|---|---|
| N | Not suitable |
| C | May need separation. Use with caution in severe or moist environments |
| * | May cause staining, but not corrosion |
| Note: |
|
Areas on a building that seldom receive rain-washing gather salt, dust and other contaminants. When condensation, dew or humidity moistens these particles, they react with metal cladding. The reaction is often noticeable as a white zinc corrosion effect, which will precede more serious corrosion.
An unwashed or sheltered area is any surface that is above or inside a line drawn at 45° to any weathertight overhang. Such areas require special consideration, particularly in severe environments.
Unwashed areas include: unlined soffits, roof overhangs, canopies, sheltered walls, and the upper part of garage doors.
Wall claddings receive less effective rain washing than roofs, and may be harder to maintain or replace, so materials for wall cladding should be selected accordingly.
Poultice corrosion or 'under deposit corrosion' is caused when a collection of fine dust — e.g., from earthworks and quarries, or sawdust and shavings from timber processing plants — collects in crevices behind laps or flashings.
These collections increase the time of wetness and retain contaminants.
Bird droppings can contain highly corrosive and adhesive materials that will affect metallic cladding. Birds can also deposit other injurious material, e'g', fish frames and chicken bones.
Solar energy collectors, Heating Ventilation and Air-conditioning (HVAC) systems may be attached directly to the surface of profiled metal roof cladding or mounted on a frame above the roof plane.
For direct fastened solar energy collectors, a complete seal between the photovoltaic (PV) cells and the cladding is essential to prevent water ingress resulting in wet storage corrosion.
Rail mounted systems for solar energy collectors can be installed in a similar manner to HVAC systems and other roof mounted structures. Units may be laid parallel to the roof or oriented at an angle for greater efficiency and easier roof maintenance.
Support frames installed above the roof must have sufficient gap underneath
The minimum recommended gap is 100 mm from the pan, and 20 mm from the crest of a profile. A greater clearance may be required for larger installations or where greater debris deposits are expected.
The support structure may be connected to the rib of clip fixed products; or supported by the rib and attached through the profile to the purlins. The designer must ensure that the design of the roof and the structure is adequate to resist any additional wind and gravity loads associated with the panels.
Access to the roof for inspection and maintenance must be considered when designing roof mounted structures. The roof cladding should be designed to resist regular roof traffic, or the installation of walkways is recommended.
Wires and conduits penetrating the cladding must be fitted with a gooseneck or proprietary cover to prevent reliance on sealant for weatherproofness and must be installed so that their base does not impede water flow.
Support structures must be constructed of materials compatible with the roofing material and separated from the roof by a non-absorbent and non-conductive rubber gasket. Typical proprietary support structures are manufactured from aluminium with stainless steel fasteners. Contact between the stainless fasteners and the roof cladding should be avoided, but run-off is acceptable. Contact with, or run-off from, copper onto painted and unpainted steel and aluminium roofs must be avoided.
The combination of hot water and copper is detrimental to all types of roofing, and hot water from copper pipes will exacerbate the corrosive effect of copper itself.
While capillary action is both a durability and a weather tightness issue, it is considered here because it affects durability more often than water tightness, although the two are inter-related.
Capillary action is the ability of a liquid to flow into narrow spaces without the assistance of, and in opposition to, external forces such as gravity. It is caused by the combination of intermolecular forces of surface tension in the water and adhesive forces between the liquid and surrounding surfaces.
The effect can be seen in the drawing up of liquids between the bristles of a paintbrush, in absorbent materials such as blotting paper or a sponge, in a burning candle, a fountain pen, or the cells of a tree. The effect can occur in a tube, but also between two closely spaced mating surfaces.
Gravity will affect the degree of capillary action; a low sloping pair of surfaces will attract liquid more by capillary action than a vertical surface, and a narrow tube will draw a liquid column higher than a wider tube.
Capillary action is an important consideration in cladding installation and design, and can be considered in four main areas:
Roofing materials exposed to the air react with the atmosphere to form a relatively stable surface. Exposing metals to water in the absence air causes the formation of unstable surface films.
Crevice corrosion occurs in crevices and confined spaces. Crevices are often created because of overlapping flashing or sheets of cladding, or between the sheets of close stacked materials.
Design details that trap moisture, dirt, and debris should be avoided.
Corrosion can appear even with a chemically neutral electrolyte. An example of this type of corrosion is the corrosion on metals underneath paint coatings and “white rust” — the wet storage stain on closely nested zinc coated roofing sheets. Other metals, such as aluminium/zinc coated and non-ferrous metals, can suffer similar damage.
Capillary action can cause white rust to occur throughout a length of cladding
White rust on galvanised sheets causes bulky white deposits to form that can quickly lead to red rust
If end-lapping of roof sheets cannot be avoided, both ends of the lap must be continuously sealed to ensure that neither condensation run-off from the under-surface nor rainwater run-off enters the lap.
Capillary action can cause water to be drawn into closely stacked sheets, resulting in crevice corrosion or wet storage stain on both metallic coated and non-ferrous materials. On metallic-coated steel sheets, the passivation coating gives some temporary protection against this process, as do organic coatings, but longevity cannot be guaranteed for the duration of this protection. On non-ferrous, metals, wet storage stain can commence very rapidly.
Wet packs of sheets should be separated to allow surfaces to dry before substantial storage.
If wet storage stain appears on unpainted surfaces, the degree of erosion of the metallic surface may be slight despite the bulky appearance of the deposits. However, when left unchecked it, can quite quickly lead to substantial degradation. If required, measurements can be taken of the thickness of the material or the metallic coating to determine the extent of erosion.
Even if the damage is superficial, the white deposits must be removed to allow exposure to the air to allow the normal formation of stable surface films. Use a stiff bristle brush; wire brushes are not recommended as they will remove more of the protective coating.
Microcracking is microscopic cracking on the surface.
The test requirement for paint adhesion in AS/NZS 2728 and by the NZMRM is a bend test over a nominated number of material thicknesses (T) and it is measured as the internal diameter.
The radius required to avoid microcracking on metallic coated and pre-painted steel is measured externally. To obtain an external radius, add two material thicknesses to the internal diameter, and divide the result by two.
Leaf guards are widely marketed as a way to prevent the build-up of vegetable matter in spouting. They may achieve this objective, but they often result in a build-up of a plant material poultice on the eaves line of the roof.
That can cause premature corrosion of the roof by chemical reaction, greatly increased time of wetness, and prevention of adequate ventilation of the underside of the cladding.
The COP recommends against the installation of such products. Alternative solutions include fitting a durable spouting material such as copper, installing rain heads with a leaf trap, or installing a proprietary leaf-proof spouting system.
Run-off from inert surfaces such as glazed tiles, aluminium and aluminium-dominant metallic coatings, fibreglass, pre-coated metals, glass or any painted surface can cause corrosion of unpainted galvanised steel and other zinc-dominant metallic coatings. This is known as 'drip-spot corrosion' or inert catchment corrosion.
Water sitting on a surface absorbs carbon dioxide forming carbonic acid, which is reactive with zinc. On a galvanised surface, the carbonic acid reacts with the zinc and becomes neutral. On an inert surface discharging into an unprotected zinc surface, the carbonic acid is not neutralised, and reaction will be concentrated on the drip points of the inert surface onto the zinc surface.
As the formation of carbonic acid takes time to occur, inert catchment corrosion is normally seen at specific drip points of dew off a roof, rather than below rain washed painted walls and windows.
Ponding happens when water is unable to drain from a roof or gutter surface. Possible causes include lack of fall, poor penetration design, and damage to sheet ribs due to excessive spans or foot traffic. The accumulated water increases the time of wetness and can lead to poultice corrosion.
All paint systems on factory pre-painted materials are permeable to a degree and will delay, but not prevent, the corrosive effects of ponding.
Ponding can occur in gutters and spouting when joints or outlets are higher than the sole of the gutter, or when debris is left to accumulate.
To help prevent ponding, the minimum pitch for all metal roof cladding in New Zealand is set at 3°. At pitches of lesser pitches deflection of the structural members or settlement of the building can compromise drainage.
Low pitch roof spans must be sized according to the type and frequency of roof traffic to prevent ponding caused by rib damage, and penetration flashings must be of a free draining type.
Pitting corrosion is a highly localised corrosive attack that forms pits which have a very small surface area, but which can be quite deep.
Pitting occurs on non-ferrous metal when the protective passive film breaks down, or where it has been weakened or damaged by contamination. When the break-through occurs in the passive film, the actively corroding pit constitutes the anode and the large passive film surrounding the pit acts as a cathodic surface.
The rate of dissolution of the metal is strongly influenced by the ratio between anode and cathode areas, consequently the "driving force" behind pitting attack can be very strong and deterioration can spread quickly.
Swarf is the term given to the steel debris as a result of cutting or piercing a metal sheet or adjacent metal surfaces.
When cutting steel, any swarf remaining on the sheet starts corroding quickly and causes stains. These stains are often mistaken for early deterioration of the cladding.
To some degree, swarf will normally be evident at the completion of any roof cladding job. The acceptability of swarf depends on how it got there, whether techniques have been applied to minimize it, and the visual exposure of the cladding.
Light, scattered swarf is acceptable in most situations.
Swarf created by acceptable means of cutting — i.e., power drills, self-drilling screws and shears — will be either loose or lightly adhered to the surface film of painted or unpainted sheets. Most swarf can be removed by daily hosing, sweeping, or blowing which should be done at the end of each day and at the completion of the job. Avoid blowing loose swarf under adjacent cover flashings.
Any remaining swarf will not be in contact with the metallic substrate and will not cause deterioration of the roof, its effect is aesthetic only. Overly aggressive efforts to remove such swarf is likely to damage the appearance of the cladding without enhancing its durability
On highly visible surfaces, a soft rag and plastic spatula can be used to remove more tenacious swarf adhesions, or on painted surfaces, the use of a diluted mild household liquid cleaner might work. Wire brushing, steel wool, or pot scouring cloths must not be used as they will damage the organic or metallic coating.
Swarf created by unacceptable practices, such as the use of grinders and friction power blades on, or adjacent to the cladding, is often hotter on contact with the cladding. The heat may cause it to embed deeply in the organic film and be in contact with the protective metallic substrate.
Friction cutting that creates swarf can also cause heat damage to metallic and organic protective surfaces.
This can severely affect the substrate; removal is difficult or impossible to achieve without mechanically damaging the decorative and/or protective coatings.
Swarf is not the only problem that cutting with friction blades can create. Such blades will often produce excessive heat at the cutting edge, which will degrade the organic and metallic coatings.
Often roof damage is caused by sub-trades accessing the roof after installation. Roofers and other trades must be aware of how they are treating the material they are working on and the effect it may have on adjacent surfaces.
Where work is done above or adjacent to an installed roof surface, or where the roof is used a work platform for subsequent work, the main contractor must take steps to make sure the existing roof remains undamaged.
This roof shows evidence of mechanical damage to the coating, rib traffic damage to adjacent ribs within a purlin span, and excessive swarf by unacceptable cutting practices. In this case the only logical remedy was replacement.
To ensure the edge of the flashing does not mechanically remove protective coatings on the cladding, there must be enough clearance between the edge of a vertical flashing, or a notched flashing, and the cladding. Similarly, the edges of cladding running parallel to flashings, such as at a window head, should have clearance to avoid mechanical damage and allow drainage.
Having the lower edges of flashings apart from the surface they are covering helps to improve the cut edge durability of the flashing. Kick-out barge details are preferred to bird’s beak barge details for the same reason. The size of the clearance is not critical, but typically it is more than 5 mm.
Clearance is required between the bottom of profiled metal cladding and large flat surfaces. For timber-framed dwellings, E2/AS1 requires a clearance of 35 mm to an adjacent roof, 100 mm to paved ground, and 175 mm to unpaved ground.
The clearance requirements for unlined buildings are less than that required for lined buildings, as the absence of lining enables the inner face of the cladding to dry more rapidly, and inspection and maintenance of the framing can be practically achieved.
| Level 1 | Structures presenting a low degree of hazard to life and other property |
|---|---|
| Level 2 | Normal structures and structures not in other importance levels |
| Level 3 | Structures that may contain people in crowds or contents of high value to the community, or may prose risks to people in crowds. |
| Level 4 | Structures with special post-disaster functions. |
| Minimum Design Ground Clearance for Profiled Metal Cladding on Lined Buildings of Importance Level 2. | ||
|---|---|---|
| Ground Type | Minimum Clearance | |
| Garage door opening | 25 mm | |
| Walls under canopies | 35 mm | |
| Paved | 100 mm | |
| Unpaved gravel | 125 mm | |
| Unpaved lawn | 150 mm | |
| Unpaved pasture | 175 mm | |
Importance level 1 buildings may have a lesser clearance provided occupant maintenance prevents the build-up of debris against the cladding.
Greater clearance may be required where gardens abut a wall, where lawn grasses are not grazed or maintained, or where soil spillage from adjacent banks may occur. Future landscaping effects on ground levels must also be considered.
The effectiveness of clearances in achieving durability requirements is subject to the occupant ensuring that vegetation, debris, and soil do not build up against the cladding surface. Design clearance from a surface is no guarantee of durability as effective clearances are subject to site development, occupant behaviour and building maintenance.
The separation of profiled metal claddings from corrosive surfaces such as wet timber or concrete is more critical at the bottom end of cladding, where high humidity levels may be experienced for extended periods. This may take the form of a 3 to 6 mm gap, an inert self-adhesive tape or a PVC vermin strip.
Internal environments are also important, ventilation must be adequate for the building use, and absorptive of corrosive substances must not be in prolonged contact with the external or internal face of the cladding or structure.
Metals used in the roof and wall cladding industry in New Zealand are:
*Many of these can coated with an organic coating, including acrylic, polyester and PVDF.
For most of the nearly 200-year history of lightweight steel cladding, the protective metal coating has been made from zinc (usually with minor additions of other metals), and this is called galvanised steel. It works by the zinc sacrificing preferentially to the steel.
In the second half of the 20th century research looked for metallic coatings which would provide longer life. Aluminium was tried as a coating material because of its passive surface, but it was not satisfactory on its own. However, aluminium alloyed with zinc and other metals produced more corrosion proof products than any metal on its own. (See 4.4.2 Barrier Protection.)
We now have two groups of metallic coatings for steel cladding products — zinc-dominant coatings, which primarily provide sacrificial protection; and aluminium-dominant coatings, which primarily provide a barrier protective coating of aluminium oxide. Coatings containing both aluminium and zinc are now the preferred coating for roof and wall cladding products, although zinc-based coatings continue to predominate for various other products.
The composition and weights of these coatings are described in detail in AS 1397:2011. The following sections discuss metallic coatings in the order in which they appear in AS 1397, not their rate of use in the market.
Steel was zinc-coated for many years by dipping short lengths of flat or profiled sheet metal in a bath of molten zinc, and the steel was then hung to cool while the excess zinc coating drained off.
More than sixty years ago manufacturers developed a continuous hot dipping method. During the continuous hot dipping process, the steel coil is run through a bath of molten metal. The thickness is controlled by blowing off the excess coating with air jets applied to both sides of the strip as it leaves the molten metal bath.
Continuous hot dipping, as opposed to the batch immersion process, is more cost-effective and allows for greater control of the consistency, thickness, and surface condition of the metallic coating.
It is a similar process to that for continuous paint coating, shown in 4.18.1.1 The Paintline Process, with priming, coating, and ovens replaced by the molten metal tank and blow-off section.
The atmospheric corrosion performance of a hot-dipped zinc coating is closely proportional to its thickness.
The thickness of coatings in micrometres (µm) can be measured with a non-destructive magnetic induction meter or similar device which can then be converted into grams per square metre (g/m²).
There is confusion about the method of describing the coating thickness of coil-coated sheet and strip products in g/m², compared to products that were hot-dipped after fabrication. The coating thickness of sheet and strip refers to the collective amount of coating on both sides of the sheet, effectively dividing the coating weight by half. It is invalid to equate the coating weight in g/m² of hot-dipped zinc coatings on fabricated products, such as nails and screws, with that of metallic coatings on sheet and coil; the coating thickness of the fabricated products relates to one surface only.
A micron (µm) is one-thousandth of a millimetre.
Zinc Coating, commonly called galvanising, is still one of the most common metallic coating processes for steel. Galvanising describes various methods of adding a metallic zinc coating to steel to give it cathodic protection; also known as galvanic protection.
Galvanised steel is classed as a “Z”-coating and has a bold crystalline pattern or spangle, a random geometric pattern that resembles frost on a window.
There are many processes for galvanising, but only products dipped or immersed in a bath of molten zinc can be called hot-dipped galvanised, the process used for the metallic coating of steel roof and wall cladding.
The thickness of the coating can be more precisely controlled on a continuous coil galvanising-line than it can be with other methods.
The standard coating weight for unpainted galvanised coil and sheet used for roof and wall cladding is 450 g/m², designated Z450, but other coating weights are available. The coating weight for products intended for painting is 275g/m², and it is designated Z275.
Since the advent of ZM coatings, minimised spangle zinc coated products, typically used for painting, are now designated with the “M” after the weight, e.g., Z275M.
The process of zinc coating by electro-plating gives a much thinner protective film and is not considered suitable for painted or unpainted cladding materials exposed to the weather.
ZA coatings are a zinc-aluminium alloy coating consisting of 95 % zinc and 5% aluminium by mass, with the addition of lanthanides. It is commonly known as Galfan® and is as designated ZA in AS/NZS 1397. As an European product it generally conforms to EN 10214..
ZA serves the same purpose as galvanised Z and AZ coatings, but has different corrosion characteristics than both.
ZA coatings are not currently available in NZ or Australia.
ZM coatings are zinc-aluminium alloy coatings with a majority of zinc and a small amount of magnesium. Steel with a continuously hot-dipped coating of zinc with 5 -13% aluminium and 2 - 4% magnesium is designated in AS 1397:2011 as ZM. In New Zealand, it is commonly marketed as ZAM.
The coating weights are similar to Zinc coatings, with ZM 240 used for products which will later be coil coated and ZM 450 for unpainted products.
Unpainted ZM products have been used for roofing accessory and rainwater cladding applications in New Zealand, but are more commonly found in factory pre-painted products.
AZ coatings are zinc-aluminium alloy coatings with a majority of aluminium. AZ coating, marketed as Zincalume® steel, is an alloy of zinc and aluminium which is now the most commonly used coating in New Zealand for protecting steel roof and wall cladding.
AZ coating is applied in the same way as other coatings, but with a pot temperature at about 140˚C higher than galvanised coating, and it is rapidly cooled to provide a dual-phase microstructure.
The alloy consists of 50 to 60% aluminium, zinc, and a small addition of silicon. In New Zealand, the ratio is nominally 55:45. These percentage ratios are by mass; by volume, the percentage ratio changes to approximately 80% aluminium and 20% zinc. Volume is probably a more realistic measure of its nature.
The alloy coating thickness generally used for steel roof and wall cladding is 150 g/m² (AZ150). This coating is approximately the same thickness (0.04 mm) as Z275 zinc. AZ200 coatings are available as a substrate for organic coated products to be used in very severe environments.
An AZ coating protects steel both as a barrier and sacrificially, as the aluminium content provides a barrier, while the zinc content of the coating will sacrifice itself to protect the base steel.
The AZ coating is finer grained than zinc alone and has a silver matt hue with a lightly visible spangle. This finish has a relatively high level of initial reflectivity, which darkens over time.
A thin acrylic film is applied during manufacture in New Zealand. The acrylic film acts as a roll forming lubricant and minimises finger marking and surface discolouration.
Adding magnesium to an aluminium dominant zinc-aluminium alloy coating improves the cut edge corrosion resistance to a similar level as zinc coating, but still confers the improved surface protection and slower erosion rate of AZ coatings.
Steel with a continuously hot-dipped coating of 47 - 57% aluminium and zinc, with the addition of 1 - 3% magnesium by mass is designated in AS 1397:2011 as AM. It is not currently sold in NZ but is under test.
Coating weights may be AM100, AM 125 or AM150.
Stainless steel is a durable, corrosion-resistant material used in harsh environments when a non-weathering finish is desired. Chromium forms a tenacious oxide protective film on stainless steel that is transparent and self-healing, as it will repair itself on exposure to the atmosphere.
Stainless steels are resistant to most chemicals, but are subject to crevice and pit corrosion (see Wet Storage).
Some light surface staining known as tea staining may appear, but it is not damaging to the product.
Most stainless steel roof and wall cladding, flashings and panels in New Zealand are made from the 300 series of austenitic non-magnetic stainless steel, which contain chromium, nickel, and manganese, with 304 and 316 being the most common grades.
Grade 304 stainless steel is an alloy of 18% chromium and 8% nickel that provides high corrosion resistance and is known as an all-purpose alloy.
Grade 316 stainless steel should be specified where tea staining must be avoided. It contains 16% chromium, 10% nickel, with 2% molybdenum added, which increases resistance to staining and corrosion.
Grade 445 ferritic stainless steel is now available in New Zealand, which combines the corrosion resistance of grade 316 with formability approaching that of carbon steel. As the work hardening of 445 is much lower than with austenitic grades, it can be formed in a similar way to carbon steel and is more easily sheared.
Grade 445 stainless steel contains 22% chromium and 1.2% molybdenum and no nickel. It has lower thermal expansion than other grades, so it is less likely to distort in the heat of the sun. The yield stress and hardness of 445 is higher than 304 and 316, but the tensile strength and elongation properties are lower.
The corrosion resistance grade of 445 is similar to grade 316 in most marine and aggressive industrial environments.
Stainless steel is available in various mill finishes from dull matt to highly polished. The most common finishes for roof cladding and sheet metal flashings, are those designated as 2B and 2D.
The 2B finish is a bright, cold-rolled finish that is highly reflective and 2D is a dull finish that is less reflective. BA is a bright reflective surface only suitable for decorative cladding in thicker gauges. Embossed patterns are available that reduce visible distortion and minimise glare and reflection.
Stainless steel should not be cleaned with steel wool, but stainless steel wool or synthetic abrasive pads can be used. Cleaning should be done with care as roughening the surface may promote further stains.
Stainless steel fixings should be used with stainless steel sheet to avoid dissimilar metal corrosion. The fastener grade must match the grade of the cladding.
There is no well-defined yield point for stainless steels. Fully annealed or standard annealed tempers are used for ease of forming with 304 and 316 having an approximate yield strength of 290 mPa.
Austenitic stainless steels require different forming techniques than other metals, and are known to be tougher and more difficult to form than carbon steel of the same thickness, e.g.,m when shearing stainless steel the equipment capacity should be increased between 30% - 50%. Because of the toughness of the metal, sharp cutting edges dull more quickly than when used with carbon steel.
Although stainless steel is not much harder than mild steel, increased power is necessary to form it because of its high ultimate strength and its higher work hardening rate. As most forming machines are rated for the heaviest gauge steel this capacity should be de-rated by 40%.
Precautions should be taken not to contaminate the surface of the metal by inclusions from roll forming or folding equipment. It can appear as rust spots on stainless steel, which is detrimental to performance. Stainless steel coil and sheet can be supplied with a strippable film on both faces to avoid this contamination.
The aluminium alloys used in New Zealand for roof and wall cladding are included in the 5000 series.
Following strain-hardening of aluminium alloys, tempering increases the ductility by low-temperature heating, and their description regarding hardness relates to the last number, e.g. H12 or H32.
The description of tempers given to aluminium alloys can be confusing because the different alloys are strain hardened in different ways. As a result, different alloys with the same hardness description may have significantly different yield strengths.
Pure aluminium (99%) can be used as a soft edging for ridge or apron flashings required to act as a wind barrier.
Aluminium alloys are available in three surface finishes.
The high reflectance and emissivity of unpainted aluminium can reduce heat transmission considerably.
Aluminium develops a thin oxide film on the surface that is impermeable to most airborne contaminants, except for strong alkalis and acids.
| Note: Typical stocking of 5052 alloy, H36 allows for rollforming both corrugate and trapezoidal profiles. Trough section may require H34 material. H36 material can be used to manufacture most flashings, except those requiring soft edging or hemming. | ||
| Alloy; | Yield Minimum | Typical Use |
|---|---|---|
| 5005 – H32 Quarter Hard | 85 | Lockseam |
| 5005 – H34 Half Hard | 105 | Folding |
| 5052 – H32 Quarter Hard | 160 | Lockseam |
| 5052 – H34 Half Hard | 180 | Folding and curving |
| 5052 – H36 Three Quarters Hard | 200 | Rollforming and folding |
| 5052 – Fully Hard220 | 220 | Rollforming |
Zinc is a traditional roof cladding material and weathers to a dark grey patina after environmental exposure, like galvanised steel, except there is no spangle effect on the surface. Zinc roof panels and flashings are commonly 0.7 mm thick, although heavier gauges can be used. Zinc roofs are usually fully supported on sarking.
The staining potential of zinc run-off onto other surfaces is less than that of copper and lead. Flat zinc panels must be adequately vented from underneath and are available with a high-build lacquer coating to help prevent corrosion of the under-surface.
Zinc has approximately twice the thermal expansion coefficient as steel, so allowance for expansion must be made accordingly.
Under 7 °C the metal becomes brittle and is difficult to form without fracturing.
Zinc used for roof cladding generally contains small percentages of titanium and copper, which add to the properties of pure zinc.
Zinc is also available in a range of pre-patinated surfaces.
Copper is a naturally durable product.
Copper grades 122, 110, and 102 may be used in construction. Grade 122 is the most commonly used; it has been deoxidized with phosphorus, which makes it weldable. The other grades cannot be welded but can be soldered.
Copper darkens in reaction to the atmosphere. Near seawater or industrial sources of sulphur-containing gases, a green patina may develop in time. In different environments, the weathered colour may vary from dark brown to almost black.
Copper-containing alloys, such as brass and bronze, are available when different colours are requested or for accessories requiring greater strength.
Copper is more malleable than steel sheets, and annealed copper is used for hand folding or where a high degree of formability is required. Roll formed roofing, wall cladding, gutters, spouts, and flashings are typically made from half hard copper. Copper roofs are normally fully supported on sarking.
Copper must be protected from contamination when being processed with tools that have been used to process other metals because the resulting inclusions might cause pit corrosion.
Neither copper nor run-off from copper should come in contact with less noble metals, as it will cause galvanic corrosion. Avoid installing copper in contact with or receiving run-off from bituminous material or other acidic surfaces, because it prevents the formation of the protective patina, causing discolouration and a shortened lifespan.
Historically, lead has been a popular choice for roof cladding and flashings, because it is naturally durable and is easily shaped using hand tools at ambient temperatures, without the need for softening or annealing.
Lead has an inherent lack of mechanical strength and is laid on solid sarking. It has high thermal movement and, over time, there is a risk of distortion and lead sheet cracking. Sheet lead is available in weights from 6 kg/m² to 40 kg/m².
The thinner the lead, the shorter the length should be. A maximum length of 1500 mm or less than 1.5 m² is ideal.
Run-off from a new lead roof can stain other metals with a white lead carbonate. Application of a proprietary product or boiled linseed oil and mineral turpentine mixture can avoid that happening.
A factory applied cured coating that inhibits the contact between lead and oxides with water is available. The lack of contact reduces the potential for run-off staining other metals, and of lead entering ground water systems. Avoid using lead roofs to collect potable water.
Translucent sheeting should be manufactured from naturally durable products or have a protective surface film to avoid ultra-violet degradation. (See Natural light for more information.)
GRP is a composite material made up of polyester resin, reinforced with glass fibres. It is protected from UV erosion by a surface coating consisting of a gel or laminate. The composite is extruded and set over forming moulds to match specific roofing profiles.
GRP can often be used as translucent sheeting or it can be supplied with a clear, or opaque gel coat to the weather surface to provide a high level of corrosion resistance to aggressive atmospheres, where coated metal or even non-ferrous metals may not perform as required.
Examples of use can be found in extreme environments such as wool scouring plants, fertilizer stores, tanneries, acid plants and smelters, abattoirs, compost plants, galvanizing plants, and buildings in harsh geothermal areas.
Where an entire roof is clad in GRP, (rather than individual sheets separated by metal sheets as in a typical case of translucent sheeting), it affects the trafficability and safety requirements. If a GRP roof is required to be accessible, it can be manufactured incorporating woven roving reinforcement into the resin matrix to make it trafficable. Another option would be to install stainless steel safety mesh under the roof cladding, if required.
As a rule, a fastener should be no less durable than the material it is fastening. With secret fixed fasteners over a benign internal environment, this is easily achieved.
The potential for air-borne salt ingress into the cavity of a tray roof is low; the pan is flat or nearly flat, the ribs are narrow and closed at the ends. The BRANZ publication, Research Now: Roof Ventilation 2, tested the durability of metals inside a vented and unvented ceiling space. Even in this Extreme Marine environment, the corrosion rate was low. Their conclusion was that the installation of ventilation openings to roof cavities should not have a negative effect on the long-term performance of galvanised metal fixtures in these spaces. In the unvented spaces, the corrosion rate was even lower.
Compatibility of materials should also be considered. Aluminium, zinc, and aluminium/zinc alloys are compatible in contact. While not normally considered compatible, stainless steel clips have also been proven to be durable, and not affect the durability performance of aluminium, zinc, aluminium/zinc alloys, or copper, in a dry internal environment.
Over wet and corrosive internal environments, or where the underside of a roof or canopy is exposed to weather from underneath, both the durability of the clip and compatibility of the roof and cladding material must be assessed in accordance with the micro-environment.
In all cases, adequate ventilation of the roof or wall is essential to prevent degradation of the cladding and other building elements.
| Roof | Galvanised, Zincalume | Pre-painted Steel | Pre-painted aluminium | Zinc | Copper |
|---|---|---|---|---|---|
| Clip | Galvanised, | Galvanised, | Painted galvanised, stainless steel, plastic | Stainless steel, Plastic | stainless steel, Plastic |
The performance of metallic coated profiled metal can be enhanced by the application of an organic (“paint”) coating. In pre-painted steel coil this is applied continuously as a two-part primer/topcoat system, prior to the material being roll formed into profile.
The different combinations of metallic coating/primer/topcoat all have different characteristics and must be matched to the material and the environment in which they are to be exposed.
The primer and the topcoat have different performance requirements to fulfil.
The purpose of the primer is to adhere to both the substrate and the topcoat, and to give added corrosion protection. Primers used on coated steel coil have anti-corrosive pigments which inhibits corrosion through an electro-chemical reaction.
This is the outer skin and it must give the desired appearance. In terms of durability, it provides a measure of barrier coating while still being breathable, and prevents UV degradation of the primer.
Functionally the top coat must be hard enough to prevent excessive marring during profiling and installation; and when in use, it must:
More recent innovations include solar reflectance technology to minimise the amount of solar heat gain gathered by the cladding
Backer coats generally have the same primer and a thinner top coat than the upper surface. Double-sided systems can be specified for areas where the underside is seen, but in external environments these will be exposed to salt spray and other contaminants, and it cannot be assumed that the underside will last as long as the rain washed top surface — even with regular maintenance.
As paint formulations from different suppliers may have different performance characteristics, it is important that cladding and accessories are supplied from the same manufacturer as differing weathering characteristics may result in visible variance in appearance.
Surface coating must comply with AS/NZS 2728. Cladding and flashings must come from the same source and have the same coating specification, so that fade rates are similar. Fade rates must not exceed those stipulated in AS/NZS 2728. All coatings must be lead free and suitable for the collection of drinking water.
AS/NZS 2728 has been deemed ambiguous in that it can be interpreted as allowing accelerated testing to determine colour fastness and durability. Such tests have been found to be an unreliable indication of a system’s performance in real-life situations. MRM has therefore adopted an interpretation of this standard as the compliance standard for its members. In the MRM standard, the four-year real-life testing for durability and colour fastness are clarified as being Normative (Compulsory).
This coating system is most commonly used for pressed metal tiles.
Where a coating is applied to a Metallic coated steel or Aluminium substrate after the profile of the roofing sheet has been formed is referred to as a Post-Painted Factory Applied Finish. The metallic substrate may have a coil coated primer, coil coated anti-finger print coating, a factory applied post painted primer or be cleaned and treated to suit the application of the coatings in a factory environment.
Post Painted coating applied to the substrate vary from smooth matt or gloss painted finishes and textured granule finishes bonded to the substrate with high build coatings. The dry film builds are high compared to pre-primed or coil coated products and the films seal any micro-cracking of the metallic coating that may have occurred in the forming process.
The corrosion performance of the post painted factory applied finished products is influenced by the substrate. The use of Aluminium Zinc metallic coating (AZ) has been proven to work extremely well with the post painted finishes in almost all environments.
The coatings are durable with expected first time recoating maintenance of over 15 years. Touch up of these coatings is possible as the same coating formulation used can be applied in the field. As with any organic coating there will be a gradual breakdown of the resin systems which may result in chalking of the surface.
Chalking is the result of erosion of the surface coating and slight colour changes may occur as the surface resin is eroded. The extent of change depends on pigments used and also the gloss level, a coating that started out as a matt finish will change very little while a gloss finish will appear to have changed more.
Granule-textured finishes use crushed rock that is either natural or natural rock with the surface coated with a ceramic coating that incorporates light-fast inorganic pigments. Both the natural rock and ceramic coated granules provide exceptional long term colour durability. The coatings used to adhere the granules to the substrate are designed to be flexible in all environments and, although durable, the coating is protected from UV by the UV opaque granules.
A clear coating is applied to the granules during the coating process, which helps bond the granules into the adhesive base coating and provides a robust surface reducing any damage during the transportation and installation process. As this clear coat weathers, it exposes the natural or coated surface of the granules, which is slightly duller than the initial finish.
Powder coating is generally used to colour match accessories used with pre-painted steel cladding and rainwater goods. The use of powder coating on metallic coated steel roof and wall cladding and flashings is not recommended for the following reasons.
Power coating may be used to colour match accessories but cannot be relied upon to increase durability, unless specific pre-treatment and powder coating systems are specified and applied.
End laps should be avoided where possible. Side laps on profiled sheets do not require priming as they are designed with capillary grooves to drain naturally, or other means to avoid the accumulation of condensation or rainwater.
All cladding fasteners must be compatible with the material, suitable for the environment and a durability equivalent to that of the cladding material. All exposed fasteners must have a minimum durability of Class 4. (See 17 Testing and MRM Standards)
Only aluminium or stainless steel screws and washers should be used on pre-painted aluminium roof and wall cladding. Stainless steel fasteners must not be allowed to come into contact with the cladding and should be installed through oversize holes.
Sealing washers must be non-conductive to prevent electrical contact between the screw, the metal washer and the cladding surface.
Steel cladding screws can be subject to hydrogen embrittlement when they are hot dipped galvanised. Alternative methods of galvanising, such as peen plating and other metallic coatings, are generally used in combination with an organic coating.
Care should be taken to minimise damage to the head of the nail or screw when using colour matched painted hot-dipped galvanised nails, bolts and screws.
Sandblasting in exposed conditions can significantly reduce the coating thickness and the longevity of the fastener.
Premature failure can result when the shanks of the screws and the eaves purlin are exposed to sea spray, and a high-fronted gutter is recommended to help prevent this.
The performance of the shank of the fastener is also affected by internal environments when the contaminant is inside the building, e.g., animal shelters, fertiliser works.
All fasteners should be easily identified by a code stamped on the head to identify the manufacturer and the coating class.
To avoid damage to the coating to the screw-head, drivers should not be of the impact type and should be fitted with a snug fitting driver bit.
Underlay should have durability no less than the cladding material, and be compatible in contact with the roofing material.
Galvanised wire netting or mesh can be used to support underlay where the internal environment is not aggressive, but is not to be used with painted aluminium.
Plastic mesh, tape or string may be used to give support at a maximum of 300 mm centres. These alternatives should be used when underlay support is required with aluminium cladding. Ensure that the underlay-support fasteners do not come in contact with the underside of the cladding.
For buildings with harsh internal environments, stainless steel, or PVC-coated mesh and netting are available. PVC-coated mesh may suffer degradation from UV radiation if used externally or when it is exposed to high levels of direct or reflected sunlight.
